Metal enrichment of particulate organic matter in arable soils with low metal contamination

Abstract

Particle-size fractionation of soil samples followed by the analysis of metal distribution in soil organic matter (SOM) fractions constitutes a promising approach for the studying of SOM-heavy metals interactions, as it isolates soil fractions that relate to dynamics and functions of SOM in situ. Metal-enrichment of soil particulate organic matter (POM) was shown in recent studies to occur in various soil–plant systems. The aim of the present study was to investigate whether it extends to arable soils where metal contamination is low, and where organic inputs to the soil are not initially metal-enriched. We studied two arable soils located about 1 (S1) and 3 km (S3) from former industrial plants in France. Both showed low soil contamination by Zn, Pb and Cd, as a result of past atmospheric deposition. Metal concentrations in POM fractions from both soils were higher than in the bulk samples, and increased with decreasing POM size. POM fractions of decreasing size (>2, 2–0.2, 0.2–0.1, and 0.1–0.05 mm) exhibited concentrations that were 3–8 times larger than those of bulk soil samples for Zn, 1–7 times for Pb, and 5–11 times for Cd. In the finest POM fraction (0.1–0.05 mm), metal concentrations reached approximately 1000–1800 μg Zn g −1 fraction, 400 μg Pb g −1 fraction, and 10–15 μg Cd g −1 fraction, in S3 and S1, respectively. Based on the percentage of soil-C present as POM-C, our study suggest that, in spite of such high metal contents, there was no reduction in crop residues mineralization. Metal enrichment of particulate organic matter is discussed in relation to the extent of POM decay and residence time in the soil.